"SSH Three frames" Hibernate part eighth: operating-many relationship

In hibernate in the-many relationship. It is not usually used. Because when the database query complexity is too high.

What we do here are students and teachers, students can have multiple teachers, teachers can have multiple students.

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We first set up a student entity class: Student.java

Package Cn.itcast.hibernate.domain;import Java.util.set;public class Student {private int id;private String name; Private set<teacher> teachers;public int getId () {return ID;} public void setId (int id) {this.id = ID;} Public String GetName () {return name;} public void SetName (String name) {this.name = name;} Public set<teacher> Getteachers () {return teachers;} public void Setteachers (set<teacher> teachers) {this.teachers = teachers;}}

We have defined three three properties. Each is an ID, a name, and a set set

And then the teacher. Entity class: Teacher.java

Package Cn.itcast.hibernate.domain;import Java.util.set;public class Teacher {private int id;private String name; Private set<student> students;public int getId () {return ID;} public void setId (int id) {this.id = ID;} Public String GetName () {return name;} public void SetName (String name) {this.name = name;} Public set<student> getstudents () {return students;} public void Setstudents (set<student> students) {this.students = students;}}

We still define three entity classes, ID, name, and a set set

Then, let's look at the mapping file for the Stduent class:

<?xml version= "1.0"?>  <! DOCTYPE hibernate-mapping public       "-//hibernate/hibernate mapping DTD 3.0//en"      "http// Hibernate.sourceforge.net/hibernate-mapping-3.0.dtd ">  
In this file. We defined: ID self-increment, name attribute. Another <set> tag. The Name property is that property of the set collection in Student.java. Then we define the foreign key in the <key> tag as student_id, and then define a <many-to-many> tag, which specifies a many-to-many relationship.

Below is the mapping file for the Teacher.java class:
<?
XML version= "1.0"?
>  <! DOCTYPE hibernate-mapping public       "-//hibernate/hibernate mapping DTD 3.0//en"      "http// Hibernate.sourceforge.net/hibernate-mapping-3.0.dtd ">  
Almost identical to the mapping file for the Stduent class above

Then, we write a test class: Many2many.java
Package Cn.itcast.hibernate;import Java.util.hashset;import Java.util.set;import org.hibernate.session;import Org.hibernate.transaction;import Cn.itcast.hibernate.domain.student;import Cn.itcast.hibernate.domain.Teacher; public class Many2many {public static void main (string[] args) {add (); query (1);} static void Add () {Session s = null; Transaction tx = null;try{//defines a teahcer set set set<teacher> ts = new hashset<teacher> ();// Defines a set set of Student set<student> ss = new hashset<student> ();//Add a teacher 1Teacher t1 = new Teacher (); T1.setname ( "T1 name"); Ts.add (t1);//Add a teacher 2Teacher t2 = new Teacher (); T2.setname ("T2 name"); Ts.add (T2);//Add a student 1Student S1 = new Student (); S1.setname ("S1"); Ss.add (S1);//Add a student 2Student s2 = new Student (); S2.setname ("S2"); Ss.add (S2);// Here is a set property setting of two teacher t1.setstudents (ss); t2.setstudents (ss);/* * This is a set property that sets two student. If the settings above are present at the same time, an exception is thrown.   Since many-to-many relationships have been established on the top * s1.setteachers (TS);   S2.setteachers (TS); */s = Hibernateutil.getsession (); tx = S.begintransaction (); S.save (t1); S.save (T2); S.save (S1); S.save (S2); Tx.commit ();} Finally{if (s!=null) {s.close ();}}} static void query (int id) {Session s = null; Transaction tx = Null;try{s = Hibernateutil.getsession (); tx = S.begintransaction ();//The Teacher object is obtained by the ID query teacher T = ( Teacher) S.get (Teacher.class, id);//The number of students corresponding to the Teacher object is called out System.out.println ("Students:" +t.getstudents (). Size ()); Tx.commit ();} Finally{if (s!=null) {s.close ();}}}}








 
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"SSH Three frames" Hibernate part eighth: operating-many relationship